This invention relates to a Hall switch of the type including a Hall element connected to a voltage threshold detector means with hysteresis, and more particularly to a symmetrical hysteresis switch requiring the same magnitude north (or south) pole magnetic field to turn it on as required of a south (or north) pole magnetic field to turn it off.
Typically, integrated Hall switches turn on when the ambient magnetic field normal to the plane of the Hall element is in one of the two relative directions, e.g. +, and exceeds an "operate" point magnitude. Such switches turn off while the ambient magnetic field is of the same polarity and drops below the operate point magnitude to a "release" point magnitude. In other words, a plot of output (voltage) versus ambient magnetic field shows a hysteresis loop that is skewed relative to the zero field axis. Thus when there is no ambient magnetic field the Hall switch is always off.
Such Hall switches are often used as proximity sensors whereby the moving part to be sensed includes a magnet. To operate the Hall switch, the magnet must be strong enough and come close enough, that the ambient magnetic field will reach the above-noted operate point magnitude of the Hall switch. A typical commercial Hall switch of this type has a maximum guaranteed "operate" flux of 350 gauss and a minimum "release" flux of 50 gauss.
Another commercial Hall switch is guaranteed to have a maximum operate flux of +250 gauss and a minimum release flux of -250 gauss. This same Hall switch has a typical hysteresis of 50 gauss usually centered at about +130 gauss. More sensitive Hall switches, for example those including amplifiers with greater gain, exhibit greater drift of those transition points in the hysteresis loop with temperature and stress.
It is an object of this invention to provide a Hall switch having greater sensitivity without an accompanying increase in drift of transition (operate and release) points with temperature or stress.
It is a further object of this invention to provide symmetrical-hysteresis Hall switches with lower and more uniform switching thresholds.